Upon an upgrade to Ubuntu 15.04, both gtkpod and Banshee successfully sync my iPod Classic 160GB 6th gen (2nd rev), including cover art.
Just run
sudo apt-get install gtkpod
I recommend using Banshee to sync music, it has a much nicer interface and is much more stable than the gtkpod
GUI, but it uses gtkpod
in the background.
It may not be necessary to upgrade to 15.04 to do this, perhaps it would be possible to just compile the latest version of gtkpod
from here (currently on version 2.1.5) to fix the problem, but I haven't tested this yet.